Abstract:Up and down Milling is one of the important factors affecting the product surface roughness and tool life.In this paper，the characteristics of up and down milling and their effects on tool life are discussed，the features of traditional machines and CNC machines，and the application of up and down milling are also analyzed.According to the cutting tests and a large number of practical applications，the reasonable selection of up and down milling can effectively improve product quality and processing efficiency.Through the analysis of typical application examples，the selection principle of up and down milling under different situations is summarized，which can provide references for modern CNC milling.